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I have been observing an issue on our Trane XL 80 furnace where during operation, the burner cycles off frequently, but always relights within 20 seconds. Model number is TUD100B948A0, and yes, it is 20 years old this year - originally installed in 1991. More specifics to the sequence of events, and "problem". 1. The thermostat calls for heat and the furnace turns on. 2. The exhaust fan turns on and shortly after that the burner lights. 3. The limit switch (Honeywell L4064A2881) can be observed as the it heats up - it passes the 80° "OFF" set point and continues to the 120° "ON" set point, at which time the blower fan turns on. 4. The temp reported on the limit switch then backs down and hovers around 100° while the blower operates. The LIMIT OFF is set point is set to 190°, but the switch never rotates up to this level during operation. ---> **Problem** <--- 5. Out of the blue, the burner cuts off. One can hear the gas valve click off, cutting the gas supply to the burner, and see the flames go out. 6. Within 5 seconds, the control module clicks and the ignition begins to glow. 7. Within 20 seconds, the gas valve clicks back open and the burners relight. 8. The furnace continues to heat the house, noting that neither the exhaust fan nor the blower ever stopped during burner cycling. 9. Steps 5-8 repeat often (every 5-10 minutes?), until the thermostat reaches the set point and turns off the furnace. 10. Furnace shuts down as expected - burner off and exhaust fan off first (together), and the blower continues until the limit switch triggers the OFF set point of 80°. Airflow does not appear to be an issue, given that the filter was just changed and the limit switch reports ~100° during operation, yet the problem still occurs. The control box red indicator light is not flashing at all, so it appears to be 'healthy'. What is the correct order of troubleshooting at this point - inducer? flame sensor? exhaust flow? gas valve? other? And how does one go about checking these items? Voltage checks and/or ohm readings I can do. I do not own a manometer...at least not yet . Thanks, Ryan.